C. S. Ajay & K. S. Reddy

Heat Transfer and Thermal Power Laboratory

Department of Mechanical Engineering

Indian Institute of Technology Madras, Chennai - 600036

International Conference on Advances in Energy Research

IIT Bombay, Powai, 10th-12th December 2013

Optical and Numerical study of Direct Steam Generation in Parabolic Trough Collector module

by

Organization of Talk

• Introduction

• Development of Optical and Thermal model

• Numerical Investigations of Direct Steam Generation in solar parabolic trough collectors.

• Summary

• Parabolic trough collector is a

– medium temperature (100oC to 400oC) concentrated solar power technology

– two dimensional concentrator that concentrates solar radiation over a line focus

• The basic components in the Parabolic Trough Collector are

– a concentrator which a reflective mirror, bend on to a shape of a parabola

– Receiver mounted at the focus

Parabolic trough collector and DSG operation

Source: asiaenergy.net

• In a solar power plant working in DSG mode,

the boiler in conventional power plant is

replaced by solar collector

• Water from the outlet of the steam condenser

is sent through the collector and is directly

converted to steam

• The steam from the collector outlet is used to

run turbines to generate power

Development of coupled Optical and Thermal model

• A standard collector (Euro-trough) corresponding to has been considered for the simulation.

• The inner wall is applied with heat flux boundary condition, given in terms of inside heat transfer

co-efficient and free stream temperature.

• The flux distribution obtained from the MCRT code is applied outside of the absorber

• The radiative heat transfer between the receiver and the glass tube is modeled using Discrete

Transfer Radiation Model (DO) model

• The length of the DSG collector considered is 500m.

q(θ)

ql,g-a

qin=hdi(Tw-Tfi)

Glass tube

Receiver

Ql,r-g

( ) u

i

Q zH z H

m

( )u ap op lossQ z SW l Q z

• Enthalpy at a given node of

the receiver is given as,

(1)

(2)

Development of coupled Optical and Thermal model

Algorithm of thermal model

Development of coupled Optical and Thermal model

Heat transfer co-efficient in two phase region

θwet

Si

 

Sv

Sl

(3)

(4)

Stratified flow

Stratified flow

• Heat transfer co-efficient in the liquid region is given by,

0.8 0.4cb,l

,

h 0.023Re Pr lDl v

h lD

1

3,l cb l nbh h h

• Dh,l ,Dh,v are the hydraulic diameters for liquid and vapor region given by

• Heat transfer co-efficient in the vapour region is given by,

0.8 0.4

,

0.023Re Pr vv v v

h v

hD

4 (1 )hl

i l

AD

S S

4

hvv l

AD

S S

• hl and hv are obtained from these equations and are applied in the nodes

of the wet and wet and dry regions depending upon the wetting angle

-0.550.12 -0.5 0.67nb 10h =55Pr -log Pr M q

θwet,sw

φ

ξ=θwet,sw-φ

O

Ro

Si

 

Sv

Sl

Stratified wavy flow

Development of coupled Optical and Thermal model

• For annular flow the inner diameter of the receiver is assumed to be completely wet and

the uniform heat transfer co-efficient is applied inside the tube.

1

3,l cb l nbh h h

• Where Reδ is the film Reynolds no and δ is the film thickness

• ul is the superficial liquid velocity and δ is the film thickness

(5)

(6)

(7)

(8)

(9)

Mathematical formulation

Annular flow

0.6965 0.4, 0.1361Re Pr l

cb a l

kh

-0.550.12 -0.5 0.67nb 10h =55Pr -log Pr M q

4Re l

l

u

(1 )

(1 )l

G xu

(1 )

D

Sl

D

δ

Results and Discussion:

• The simulations are carried out for different mass flow rates, different irradiance conditions.

The maximum temperature of the absorber tube is limited to less than 450oC.

• Five irradiance conditions which were considered are 1000W/m2, 850W/m2, 600W/m2,

400W/m2, and 200W/m2.

• The collector is also analyzed for different receiver thicknesses (5mm, 7mm, 10mm) to

study its effect of thermal gradient under two phase conditions

• The collector position are varied from 0 to 50o to study the asymmetric temperature profile

resulting from stratified flow conditions

• The position of the water-steam interface with respect to the flux distribution at different

collector positions is shown in figure

Collector position : 0o Collector position : 50o

q’ q’

V

L

V

L

Results and Discussion

Temperature distribution around the absorber

Results and Discussion

Non-dimensional temperature distribution around the absorber (thickness =10mm)

Operating conditions:

m=1kg/s; DNI= 1000W/m2;

Collector position = 0o (solar noon)

Operating conditions:

m=1kg/s; DNI= 1000W/m2;

Collector position = 50o

•Non-dimensional temperature profile (Tw-Tmin(z)) obtained around the receiver for the whole entire

collector length

Tw

-Tf

Angular position around the receiver (degrees)

Collecto

r length (m

)

Tw

-Tf

Angular position around the receiver (degrees)

Collecto

r length (m

)

Results

Non-dimensional temperature distribution around the absorber (Thickness =10mm)

Collecto

r length (m

)

Angular position around the receiver (degrees)

Non

-dim

ensi

onal

tem

pera

ture

gra

dien

t (K

)

Operating conditions:

m=0.155kg/s; DNI= 200W/m2;

Collector position = 0o (solar noon)

Operating conditions:

m=0.155kg/s; DNI= 200W/m2;

Collector position = 50o Tw

-Tf

Angular position around the receiver (degrees)

Collecto

r length (m

)

Tw

-Tf

Angular position around the receiver (degrees)

Collecto

r length (m

)

Results and DiscussionCross-sectional view of receiver at the point of maximum thermal gradient in the sub-cooled,

superheated and stratified two phase regions

Collector inclination = 0o

Collector inclination = 50o

Sub-cooled region Stratified region Super heated region

Collecto

r length (m

)

Angular position around the receiver (degrees)

Non

-dim

ensi

onal

tem

pera

ture

gra

dien

t (K

)

Collecto

r length (m

)

Angular position around the receiver (degrees)

Non

-dim

ensi

onal

tem

pera

ture

gra

dien

t (K

)

Results

m=1kg/s; DNI= 1000W/m2;

Collector position = 0o (solar noon)

m=0.155kg/s; DNI= 200W/m2;

Collector position = 50o

Non-dimensional temperature distribution around the absorber (thickness =5mm)

m=0.155kg/s; DNI= 200W/m2;

Collector position = 50o

Tw

-Tf

Angular position around the receiver (degrees)

Collecto

r length (m

) Tw

-Tf

Angular position around the receiver (degrees)

Collecto

r length (m

)

Tw

-Tf

Angular position around the receiver (degrees)

Collecto

r length (m

)

Results and DiscussionCross-sectional view of receiver at the point of maximum thermal gradient in the sub-cooled,

superheated and stratified two phase regions

Sub-cooled region Stratified region Super heated regionCollector inclination = 0o

Collector inclination = 50o

Conclusion and Summary

• A coupled MCRT-DSG model has been developed for finding thermal performance characteristics of

the DSG collector

• Thermal analysis on the receiver revealed that the thermal gradient higher in the stratified flow and

this effect is higher at higher collector inclination

• The collector inclination increases the temperature gradient across the cross-section by 18%

• The maximum temperature gradient across the receiver cross-section in the DSG collector is

obtained in the stratified region as 1022 K/m at an irradiance of 200W/m2